1. Field of the Invention
The present invention relates to an analog-digital code converter in a digital telephone switching system wherein a wide band communication signal and a narrow band communication signal supplied from a common analog line are analog-digital converted and selectively connected to a time division multiplex speech path switch through a path having a plurality of frequency cut-off characteristics, one of which is selected depending on the type of the communication signal.
2. Description of the Prior Art
Where a time division digital switching system is to be introduced in a subscriber stage of a telephone network, it is necessary to install an analog-digital code converter in the subscriber line. Usually, it suffices to consider a voice frequency signal band up to 3.4 KHz. Taking a most typical PCM telephone switching system as an example, signal band components which are higher than 3.4 KHz are cut-off by a low-pass filter in an analog-digital code converter and the remaining components, i.e., those within the pass band, are sampled at 8 KHz and a predetermined coding rule is applied thereto to convert them to a PCM signal.
In such a primitive analog-digital code converter, however, a sampling rate is low and hence a higher rate is desired. One of the biggest problems is the necessity of providing a 3.4 KHz cut-off characteristic filter for each analog per line unit. This is very expensive.
In an approach to overcome the above problem, in a known analog-digital code converter employing a delta modulation PCM system which has been recently recognized to be advantageous with the development of digital integrated circuit technology and large scale integrated circuit technology, broadening of the band width is attained by increasing the sampling rate for modulation and demodulation in the analog per line unit (e.g., to 32 KHz instead of 8 KHz used in the prior system). This approach intends to attain economy by allowing a rough cut-off characteristic for the low pass filter used in the analog per line unit. In a common unit in which digital multiplexing and demultiplexing are performed, the high speed sampling digital code (32 KHz PCM) is passed through a digital filter to effect rate-conversion to produce a low speed sampling digital code (8 KHz PCM), which in turn, is communicated to a time division digital switch to perform time slot exchange. Accordingly, in spite of the fact that the analog per line unit of the analog-digital converter handles the high speed code (i.e., wide band signal), only the 8 KHz PCM (narrow band) is handled as the interface to the time division switch. As a result, the signal pass band is limited within 3.4 KHz. Although this does not raise a problem in ordinary voice switching, it does not allow wide band communication service such as broadcasting service which requires a signal pass band of up to 7 - 8 KHz. In other words, since the prior art analog-digital converter has been designed in circuit configuration so as to be most economic for the ordinary speech exchange, it cannot pass the signal band of above 3.4 KHz and lacks the expandability to additional services.